In a recording medium, such as a DVD and a CD (Compact Disc), information is recorded as a pit row on the recording surface thereof. For example, in the case of a recording disc, the information is recorded on a track, which is formed spirally or concentrically.
In the case of an optical recording medium or a magneto optical recording medium or the like, when the information is recorded onto the recording medium, or when the information recorded on the recording medium is read, it is necessary to accurately position the spot of the light beam, which is irradiated from a pickup, on the track. In order to realize this positioning, a recording apparatus or a reproducing apparatus is provided with a tracking control circuit for tracking control which allows the light beam to follow the track.
The tracking control circuit is generally provided with: a light receiving element for receiving reflected light of the light beam from the recording surface of the recording medium and for converting the reflected light to an electrical signal; a signal processing circuit for generating a tracking error signal which indicates a shift amount between the spot of the laser light and the track, on the basis of the electrical signal outputted from the light receiving element; and a drive circuit for displacing the position of an objective lens located in the pickup or the position of the pickup itself, in accordance with the tracking error signal.
As a tracking control method performed on such a tracking control circuit, generally, a Differential Phase Detection method is known. The Differential Phase Detection method uses a four-division light receiving element which has four light receiving units, divided by a first dividing line extending in a direction of crossing the track and a second dividing line extending in a direction along the track. Then, a sum signal obtained from two light receiving units located on one diagonal out of the four light receiving units and a sum signal obtained from the other two light receiving units located on other diagonal are obtained. The tracking error signal is generated on the basis of a phase difference between two sum signals.
In the Differential Phase Detection method, however, if a pit depth formed on the recording surface of the record medium is shifted from λ/4n, there is such a problem that offset occurs in the tracking error signal and this offset may prevent the realization of tracking control. Incidentally, λ is the wavelength of the light beam, and n is the refractive index of a cover layer of the recording medium.
Moreover, Japanese Patent Application Laid Open NO. 2001-338425 (patent document 1) discloses a tracking control method (hereinafter referred to as a “conventional tracking control method”) using the four-division light receiving element, as in the case of the Differential Phase Detection method. In the conventional tracking control method, phases of signals outputted from two light receiving units located on one side of a boundary made by the first dividing line extending in a direction of crossing the track are compared. Moreover, a phase difference signal with an amplitude corresponding to a phase difference between the signals is generated, and the phase difference signal is used as the tracking error signal.
According to the conventional tracking control method, even if the pit depth of the recording medium is shifted from λ/4n, the offset does not appear in the tracking error signal. Thus, according to the conventional tracking control method, it is possible to solve the above-mentioned problem in the Differential Phase Detection method in a sort.
Moreover, Patent Publication NO. 2716569 (patent document 2) discloses a tracking control method (hereinafter referred to as a “conventional tracking control method”) using the four-division light receiving element, as in the case of the Differential Phase Detection method. In the conventional tracking control method, a phase of a first electrical signal outputted from one of a certain pair of light receiving units located in the diagonal position out of the four light receiving units of the four-division light receiving element is compared with a phase of a sum signal of electrical signals outputted from the respective four light receiving units. Moreover, a first phase difference signal with an amplitude corresponding to a phase difference between the first electrical signal and the sum signal is generated. Furthermore, phases of a second electrical signal outputted from the other of the certain pair of light receiving units is compared with the sum signal of electrical signals outputted from the respective four light receiving units. Moreover, a second phase difference signal with an amplitude corresponding to a phase difference between the second electrical signal and the sum signal is generated. Then, a signal which is obtained by adding the first phase difference signal and the second phase difference signal is used as the tracking error signal.
According to the conventional tracking control method, even if the pit depth of the recording medium is shifted from λ/4n, the offset caused by this shift can be eliminated in the tracking error signal generation procedure. Hereinafter, an outline for the elimination of the offset component by the conventional tracking control method will be explained. Namely, if the pit depth of the recording medium is shifted from λ/4n, the phase of the first electrical signal proceeds in accordance with the shift amount of the pit depth with respect to the phase of the sum signal. As a result, the offset component (e.g. a minus amplitude component) corresponding to the shift amount of the pit depth appears in the first phase difference signal. Moreover, if the pit depth of the recording medium is shifted from λ/4n, the phase of the second electrical signal delays in accordance with the shift amount of the pit depth with respect to the phase of the sum signal. As a result, the offset component (e.g. a plus amplitude component) corresponding to the shift amount of the pit depth appears in the second phase difference signal. Then, if the first phase difference signal and the second phase difference signal are added to each other, the two offset components corresponding to the shift amount of the pit depth is canceled. As described above, according to the conventional tracking control method, it is possible to eliminate the offset caused by the shift of the pit depth in the tracking error signal generation procedure, and it is possible to solve the above-mentioned problem in the differential phase detection.    Patent document 1: Japanese Patent Application Laid Open NO. 2001-338425 (FIG. 3)    Patent document 2: Patent Publication NO. 2716569